The present invention relates to a multiplex transmission method used in, e.g., a vehicle and, more particularly, to a distributed control type multiplex transmission method, wherein each communication node performs a refresh transmission operation during a corresponding transmission period, and has a clock to manage its own transmission timing based on the clock.
In recent vehicles, the number of electronic devices equipped in a vehicle is considerably increased along with the development of electronic control, and this results in an increase in the number of wiring lines for connecting the electronic devices, and a complicated wiring structure. In order to solve these problems, as disclosed in, e.g., Japanese Laid-Open Patent Application No. 61-224634, a multiplex transmission method has been examined, and is beginning to be put into practical applications. In this method, signal transmission operations among a plurality of electronic devices are performed in a multiplex transmission mode, i.e., communication nodes of the plurality of electronic devices are connected to a common transmission path, and signal transmission operations among the communication nodes are performed in the multiplex transmission mode.
As a multiplex transmission method of this type, the following time-divisional distributed control type multiplex control method is known. More specifically, each communication node performs a so-called refresh transmission operation, i.e., transmits a signal for informing its own information at every predetermined transmission period, and transmits its own information at every predetermined period regardless of a change in its own information, thus assuring reliability of data. At the same time, each communication node performs a so-called event transmission operation for, when its own information changes, immediately informing the changed information, as needed. Each communication node has its own clock, and manages its own transmission timing based on the clock.
As described above, in the distributed control system wherein each communication node has its own clock, and independently manages its own transmission timing according to the clock, a control program is relatively simple, and a communication node can be easily added.
However, the refresh transmission operation of the above-mentioned distributed control system suffers from the following problem. That is, the refresh transmission interval is decreased due to an error among clocks of the communication nodes, and signals are crowded on a transmission path, resulting in an increase in traffic amount. As a result, a transmission disabled or delayed state occurs due to signal collision.
More specifically, in the refresh transmission mode, a predetermined time interval is initially set between refresh transmission timings of the communication nodes, and each communication node repeats the refresh transmission operations at the predetermined transmission period on the basis of its own clock.
In this case, if the clock is perfectly precise, the time interval can always be assured, and no problem is posed. However, each clock has limited precision, and suffers from a time measurement error. For this reason, the refresh transmission timings approach very close to each other, or refresh transmission signals collide against each other several hours after the multiplex transmission operation is started.
When the refresh transmission timings approach each other, and the traffic amount is increased, not only refresh transmission signals collide against each other to cause a transmission disabled or delayed state, but also when an event transmission operation must be performed when the traffic amount is increased, the event transmission operation also suffers from a transmission disabled or delayed state due to collision.
Furthermore, when an event transmission operation is performed immediately before a refresh transmission operation, the interval between the event and refresh transmission operations is decreased, and the traffic amount is increased, thus causing a transmission disabled or delayed state of another event transmission.